The dependency-install fallback chain unconditionally ran
'pip install --user', which fails inside a virtualenv (and as root in
LXC/containers) with 'Can not perform a --user install. User site-packages
are not visible in this virtualenv.' — even though the function's docstring
already noted --user is invalid in venvs.
Guard the --user fallback with a venv check so it only runs outside a venv
(where --user is actually valid for PEP-668 system Pythons). Derive the venv
probe interpreter from the install command (python for 'pip', python3 for
'pip3'/'python3 -m pip') so the check runs in pip's own environment. System
PEP-668 installs keep the --user fallback; venv/LXC-root installs no longer
hit the --user error. Updated the unit test for the new chain.
Closes#388
9a1893760d